The conventional bonding technology used to connect the actual circuit chip to external leads is wire-bonding, either manual or automated. As is known, a fine wire is pressed against the material of the bond pad with pressure and/or heat used to effect a weld between the wire material and the metal on the bond pad. The wire bonding technology is quite well known and the actual bonding process is not part of this invention.
It is known in the art that a mis-bonded wire may be pulled off and the bond done again if there is sufficient room on the pad. In commercial applications, the restrictions on rebonding are generally technical--that the pad is not damaged or that there is sufficient undamaged area to make a new bond. In the particular field known as the aerospace market, standards generally conform to those set by the U.S. Government. The government provides detailed standards for acceptance of wire bonds and also for the amount of the rework that may be done to replace a defective bond. According to MIL-STD 883, for example, a bond that has been reworked is not acceptable if there is: "A bond on top of another bond, bond wire tail or residual segment of lead wire. An ultrasonic wedge bond alongside a previous bond where the observable width of the first bond is reduced less than 0.25 mils, is considered acceptable.", so that the location of the first bond can affect the success of a second bond.
Further, there are standards set by the government and also in conventional practice for the distance of closest approach between a wire extending from a pad and the adjacent bond pad, and for the distance of closest approach between two wires. This distance may be simply zero, or it may be any desired amount, depending on the customer's desires.
As integrated circuit technology has improved, the pitch, or spacing between corresponding points on adjacent bond pads, has constantly decreased in order to accommodate the use of smaller chips and greater numbers of input/output contacts. Consequently, it has become more and more difficult to make the bonds while still complying with the applicable standards. Conventional practice is not well adapted to reworking bonds and still staying within the permitted allowance. Rejected circuits mean an increase in the average cost of good circuits, of course.
According to conventional practice, the first bond on a pad is made in the nominal center of the pad, because that location permits the greatest chance that the first bond will be successful since the center has the greatest distance from each edge. When this is done, however, the second bond must be located close to the edge and, therefore, the second bond has a high chance of violating the tolerance standards.